The present invention relates to an objective lens for recording and/or reproduction of information for an optical information recording medium, an optical pickup device including the objective lens, and a recording/reproduction apparatus including the optical pickup device.
Novel high density recording optical pickup systems each employing a light source such as a violet semiconductor laser or a violet SHG laser having an oscillation wavelength of about 400 nm and an objective lens whose numerical aperture is raised up to about 0.85 are under studies and developments recently. It is known that the recording density of an optical recording medium such as an optical disk and a magneto-optic disk grows greater in inverse proportion to an area of a spot formed on an information recording surface through light-converging by an objective lens (=k·(λ/NA)2), wherein k represents a proportion constant, λ is a wavelength of a light source and NA represents a numerical aperture of the objective lens). Though a method to achieve high density of an optical recording medium includes a method to shorten a wavelength of a light source, in addition to a method to make a numerical aperture of an objective lens to be higher, the method to shorten a wavelength of a light source has a problem that it is impossible to obtain the light utilization efficiency that is practically sufficient, because internal transmittance of a lens material falls sharply in an area of the wavelength shorter than 350 nm. It is therefore estimated that an objective lens is required to have the numerical aperture that is further higher, for higher density in a novel optical pickup system.
Though the high density recording optical pickup system mentioned above is equipped with a two-group-structured objective lens, an increase of man-hour for assembling a first lens and a second lens, deterioration of production efficiency and a cost increase are caused, because eccentricity tolerance between lenses is small in the two-group-structured objective lens having high NA.
Further, since depth of focus d of the objective lens is in inverse proportion to a square of the numerical aperture, the greater the numerical aperture is, the smaller the depth of focus is, and the speed of response and accuracy required for an actuator for focusing of the objective lens become higher increasingly. Since a two-group-structured objective lens is greater than a single lens in terms of weight, it causes a large-sized actuator and an increase of cost.
Further, an working distance of the two-group-structured objective lens having high NA tends to be small, compared with a single lens. In the design of an objective lens with high NA, security of sufficient working distance is an importance matter in a sense of preventing damages of an optical recording medium.
From the foregoing, it is preferable that the objective lens with high NA is a single lens which does not require assembling while is light in weight and makes it possible secure a sufficient working distance.
Incidentally, in the case of an aspherical single lens for recording and/or reproduction for an optical recording medium, spherical aberration and coma are corrected by an aberration correcting action of an aspheric surface, but when NA grows greater, astigmatism as wavefront aberration grows greater and image height characteristics are deteriorated, which is a problem.
When the depth of focus of the objective lens becomes smaller, defocus component generated when instantaneous wavelength variation which cannot be traced by focusing of the objective lens takes place on the light source, grows greater. It is therefore necessary to correct chromatic aberration more strictly when the numerical aperture grows greater.
An object of the invention is to provide an aspherical single objective lens for recording and/or reproduction of information for an optical information recording medium wherein the numerical aperture is made to be greater than 0.85, and image height characteristics are excellent.
Another object is to provide an aspherical single objective lens which is suitable to be used for a high density optical pickup apparatus employing a light source whose wavelength is as short as about 400 nm, and has a numerical aperture that is made to be greater than 0.85.
Further object is to provide an aspherical single objective lens having a numerical aperture greater than 0.70 wherein image height characteristics are excellent, a working distance is secured sufficiently, and a weight is less.